XF 1.5Members Online Now getting a reset?

Member

I would like to now the reason if the Member Online Now turns Zero from one to another moment like a reset. It was quiet strange, from 81 Guest to 0 Guests, and then they came slowly back. Something in that way happend never before and xenforo ist running like charme since a year. So is it possible that we are compromised or what reason can cause this effect? The board is hosted on a shared Server.